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Techno Cases
Techno Fysica BV is a
Dutch-based engineering
company, highly specialised in
troubleshooting and
solving all kinds of problems related to the dynamic loading of machinery
and installations. For this, several measuring and analysis techniques are
used. In the article below, we give an example of one of many problems
related to torsional vibrations, which exist in any installation powered
by a combustion engine. In general, these
vibrations should not be harmful.
However, in some situations, the can lead to serious
problems resulting in damage and down-time of the installation.
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Heavy damage to the
PTO of the gearbox of a
trailing suction hopper
dredger (TSHD)
as a result of excessive torsional vibrations |
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On
a TSHD (trailing section hopper dredger), repeated damage occurred to the
gearbox between the main engine and the propeller shaft.
In
addition to a propeller shaft, this gearbox was
also equipped with a 4000 kW PTO generator for the dredge pumps.
The
damage consisted of cracked cages of the PTO roller bearings, fretting of
the fit between the input shaft and the PTO wheel and axial movement of
the Geislinger coupling between main engine and gearbox which caused
excessive load on the axial bearing of the crankshaft.
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Gearbox of a TSHD which suffered from damage due to axial vibrations, brought about by torsional vibrations. |
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The
installation was equipped with torque meters on the PTO shaft and on the
propeller shaft, a laser torsiometer on the engine flywheel, proximity
probes for the axial position of the PTO shaft and the input shaft and a
sensor to detect the position of the fuel rack.
Measurements were carried out during dredging, free sailing and shore
discharge conditions.
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Dynamic behaviour as measured on the shafting system of a
TSHD during unwanted interaction between governing system and torsional
vibrations. |
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From the measurements it became clear that also on this ship, interaction
between governing system and torsional vibration behaviour occurred.
Under certain combinations of low to moderate propeller
load and low load on the PTO generator, this interaction resulted in
vibratory torque amplitudes of up to 50% of nominal torque.
Because this gearbox is equipped with inclined teeth,
changes in torque also result in changes in axial force that lead to axial
vibrations of the PTO shaft and the input shaft.
This mechanism explained the damages that had occurred.
This problem was cured by changing the dynamic torsional stiffness of the
PTO coupling which shifted the resonance speeds of the installation to an
engine speed below the low idle speed of the engines.
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For
more info, please contact techno Fysica BV in the Netherlands at:
Techno Fysica BV
p.o. box 351
2990 AJ Barendrecht
The
Netherlands
Tel. +31 180 620211
Fax.
+31 180 620705
info@technofysica.nl
www.technofysica.nl
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